Study On Mechanical Properties Of Plastic Pipe Reinforced By Cross Helically Wound Steel Wires Under Combined Loads Of Internal Pressure And Bending Load

Plastic pipe reinforced by cross helically wound steel wires(PSP)is a new type of composite pipe.A two-layered steel wire mesh skeleton is sandwiched in the middle with high-density polyethylene(HDPE).The inner and outer steel wires are wrapped with a negative angle and a positive angle,respectively.High-strength steel wires are integrated with HDPE using the high performance adhesive resin.PSPs are widely used in marine,petroleum,chemical and municipal fields,which combine the high strength of steel wire with the anti-corrosion of HDPE.Besides internal pressure,PSPs are subjected to complex loads in marine conditions,such as bending load caused by falling objects,ships trawling,anchors dragging,and ocean wave,and influenced by temperature.Therefore,internal pressure and bending load are typical combined loads in marine conditions.When the combined loads exceed the bearing capacity of PSP,failure occurs,and it causes great economic losses and ecological damage.However,there is no research on PSP subjected to combined loads of internal pressure and bending load,and no testing equipment for applying complex loads as well.Therefore,research on the mechanical properties of PSP under combined loads of internal pressure and bending load is necessary,which is significant for safety application and engineering design.This paper was under the support of Chinese National Natural Science Foundation:"study on the mechanism of slow crack propagation of polyethylene pipe based on viscoelastic damage model"(project No.51575480)and "study on formation mechanism of characteristic line by ultrasonic inspection of polyethylene pipe electrofusion welding joint"(project No.51305394).In this paper,the mechanical properties of PSP under combined loads were studied by experimental and finite element(FE)method.The main research works are as follows:(1)According to the complex loads of marine conditions and material properties of temperature-dependent and rate-dependent of polyethylene,the function of the equipment was proposed:the combined loads of internal pressure and bending load could be applied to the pipes,and the temperature and bending rate could be adjusted.According to the functional requirement,the equipment was designed in the aspects of mechanical structure,hydraulic circuit and testing system.In the aspect of mechanical structure,sample pipe could be automatically moved into the water tank.In the aspect of hydraulic circuit,hydraulic circuits were designed for multifunctional bending loading.In the aspect of testing system,the parameters of cylinder such as the maximum load were obtained by estimate.Testing equipment was set up according to the design.And the design of the equipment obtained the national authorized invention patent.(2)Tests under combined loads of internal pressure and bending load were carried out at 20?.The crack voice of steel wires appeared,the samples of PSP broke,and water pulled out.The outer HDPE was cut out,and ten steel wires were found to be cracked,therefore the crack of steel wire was taken as the failure criterion.According to the curves of bending moment-displacement,the limit bending moment and the failure displacement were obtained.The reduction percentage of internal pressure was taken to characterize the increasing degree of volume.Tests under combined loads of internal pressure and bending load were carried out at 40? and 60?.The limit bending moments,failure displacements and reduction percentages of internal pressure were obtained at different temperatures.The higher of temperature,the lower of the limit bending moment and the failure displacement,but the reduction percentage of internal pressure is basically unchanged.(3)According the loading and contact conditions of experiments,an FE model was established.Compared the curves of bending moment-displacement of experiments and FE results,the average relative error was 4.4%,therefore the FE model was verified.The flexural rigidities under different displacements were calculated.Combined the curves of flexural rigidity-displacement and bending moment-displacement,the bending process of PSP at different stages was analyzed,and the limit bending moment was obtained.According the conclusion of experiments and stress contour figures of HDPE and steel wires,the crack of steel wire was taken as the failure criterion of the FE model,and the failure curvature of PSP was calculated.The strain of steel wires was calculated under internal pressure,bending load,and combined loads of internal pressure and bending load,respectively,then the relationship between combined loads of internal pressure and bending load was analyzed.Finally,the limit bending load at different temperatures and internal pressures were calculated,and a binary linear regression equation was fitted.The influences of numbers of steel wires and winding angles on the limit bending moment were discussed.The models of X70 steel pipe,X80 steel pipe and PSP with different normal pressure and pipe diameters were established respectively to discuss the anti-bending abilities of different types of pipes.The analysis of anti-bending abilities of PSP and the discussion of influence factor can have guiding significance for parameter design and engineering application.